Natural enhancement and mobility of oil reservoirs by supercritical CO2 and implication for vertical multi-trap CO2 geological storage

The accumulation and production of both deep mantle-derived CO2 and light oil were discovered in the Huangqiao reservoir, which is located in the Subei Basin, East China. The Huangqiao reservoir shows that both CO2 and oil are entrapped in and produced from vertical multi-traps. The effects of deep CO2 on the accumulation and production of oil under natural conditions and the implications for CO2 geological storage are investigated in detail. The fluid inclusions in quartz or calcite veins from the Silurian Fentou Formation (S2-3f), Permian Longtan Formation (P2l) and Triassic Qinglong Formation (T1q) have homogenization temperatures (Ths) that display peak ranges of 180°C-190 °C, 170°C-180 °C and 150°C-160 °C, respectively. The Ths are higher than the formation temperatures. The calcite veins have light carbon and oxygen isotope compositions and have high 87Sr/86Sr ratios and positive Eu anomalies. These characteristics reveal the activities of deep CO2-rich hydrothermal fluids in the basin strata. The feldspar in the S2-3f and P2l sandstone reservoirs underwent significant dissolution because of the presence of CO2-rich fluids, forming of a large amount of secondary pores in the sandstone reservoirs and enhancing the reservoirs' physical properties. The measured porosity reaches 12.3%. CO2 in a supercritical state extracted or dissolved light petroleum components out of sedimentary rocks and then carried them towards reservoirs. Thus, CO2 and oil co-accumulated in the enhanced reservoirs, e.g. S2-3f, P2l and T1q. During drilling development, this supercritical CO2 naturally increased the mobility of oil, resulting in the co-production of highly pure CO2 and light oil. Based on the unique features of the Huangqiao CO2-oil reservoir, we propose a new CO2 geological storage model, namely, a vertical multi-trap geological storage model, which can either entrap large volumes of CO2 or lower the risk of long-term storage.